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Proc Natl Acad Sci U S A. Sep 30, 2008; 105(39): 14993–14998.
Published online Sep 26, 2008. doi:  10.1073/pnas.0806044105
PMCID: PMC2567481

Local BAFF gene silencing suppresses Th17-cell generation and ameliorates autoimmune arthritis


Rheumatoid arthritis (RA) is a chronic disease characterized by synovial inflammation and joint damage. Although both T cells and B cells mediate the disease pathogenesis, proinflammatory cytokines are critically involved. The TNF superfamily member B cell-activating factor (BAFF) plays an important role in humoral immunity and in autoimmune diseases, including RA. Here, we show that intra-articular injection of lentivirus expressing shRNA for BAFF gene silencing provides long-term suppression of arthritic development in a collagen-induced arthritis model. Local BAFF gene targeting inhibited proinflammatory cytokine expression, suppressed generation of plasma cells and Th17 cells, and markedly ameliorated joint pathology. Lentivirus targets dendritic cells in the joint tissue and BAFF gene silencing inhibits dendritic cell maturation and their function in driving Th17-cell differentiation in vitro. Moreover, we revealed a previously unrecognized role for BAFF in promoting the expansion of Th17 cells and demonstrated IL-17 as a crucial effector cytokine for BAFF-mediated proinflammatory effects during collagen-induced arthritis development. Taken together, these findings identify BAFF as a valuable gene-silencing target potentially for the effective treatment of RA.

Rheumatoid arthritis (RA) is characterized by chronic inflammation targeting the synovial membrane, cartilage, and bone. Both B cells and T cells form aggregates in the synovium of inflamed joints and mediate the pathogenesis of RA. Proinflammatory cytokines and chemokines produced by T cells and dendritic cells (DCs) in the inflamed joints recruit leukocytes and drive synovitis and cartilage damage (1).

Recent studies using genetically modified mice have demonstrated an important role for B-cell–activating factor (BAFF) in B-cell survival and maturation. Overproduction of BAFF in transgenic mice leads to an enlarged spleen, increased number of mature B cells, and overt manifestation of autoimmune disease resembling systemic lupus erythematosus (SLE) (2). Elevated BAFF levels have been detected in the serum of patients with various autoimmune disorders including SLE and RA, indicating a role for BAFF in these pathologies. We have previously observed a close correlation between increased serum levels of BAFF and anti-collagen type II (CII) autoantibody production in collagen-induced arthritis (CIA) mice (3). Moreover, we have found highly expressed BAFF protein in DCs during the early stage of CIA development (3). Although BAFF is a key B-cell survival factor essential for autoantibody production under autoimmune conditions, it has also been shown to costimulate T cells (4). Recent evidence indicates that Th17 cells, a newly identified T cell subset that produces IL-17, play a critical role in the development of autoimmune diseases, including RA (5). IL-17−/− mice display markedly suppressed CIA development, whereas inhibition and overexpression of IL-17 in the joints suppresses or worsens joint inflammation and destruction, respectively (6, 7). However, it has been unclear whether BAFF promotes Th17-cell generation.

Despite the incompletely understood pathogenesis of RA, extensive efforts have focused on the development of effective therapies with limited side effects (8, 9). Recently, BAFF antagonists have shown significant protection in murine models of RA, multiple sclerosis, and Graves' disease (1013). However, the extent of potential side effects from these systemic approaches remains to be assessed (14). Thus, an attractive strategy is to target the BAFF gene directly in the afflicted joints. Here, we describe the development of replication-defective lentivirus engineered to express shRNA for silencing the BAFF gene. This local gene-silencing strategy effectively inhibits synovial inflammation and joint destruction in CIA mice and can markedly suppress arthritic progression after its onset. DCs are found to be efficiently targeted by the lentivirus in situ. Moreover, we have revealed a previously unrecognized role of BAFF in promoting the preferential expansion of Th17 cells, suggesting a mechanism underlying BAFF-mediated autoimmune pathogenesis. Together, our results identify BAFF as a local gene-silencing target potentially applicable for the treatment of RA.


Intra-Articular Administration of Lentivirus Expressing shRNA for BAFF Inhibits the Onset and Progression of CIA.

To determine whether BAFF acts as a driving factor for CIA development, we injected recombinant BAFF into CII-immunized DBA1/J mice and found accelerated onset of arthritis with markedly enhanced synovial inflammation and joint damage compared with controls (supporting information (SI) Fig. S1 A and B online). Together with the significantly increased serum level of anti-CII autoantibody and number of autoantibody-secreting PCs in the joint, spleen, and bone marrow (BM), these observations indicate a pathogenic role for BAFF during CIA development (Fig. S1 C–J online).

To target the afflicted joints directly, we used lentiviral vectors expressing shRNA to silence BAFF gene for long-lasting suppressive capacity. A panel of siRNAs targeting BAFF was screened for effective gene silencing in BAFF expressing murine macrophage I13.35 cells, whereas siRNA for β-actin was used as a control (Fig. 1A; siRNA sequences are listed in Table S1 online). Corresponding short pairs of sense and antisense DNA oligo targeting BAFF or β-actin were synthesized (sequences listed in Table S1 online) and cloned into lentiviral vectors for expressing shRNAs, which markedly reduced BAFF or β-actin proteins in I13.35 cells (Fig. 1B). Administration of lentivirus expressing shRNA for BAFF (LV-shBAFF) into inflamed joints of CIA mice effectively reduced BAFF protein level in the joint tissue (Fig. 1C). We then injected LV-shBAFF intra-articularly into the hind ankle of CII-immunized mice every 48 h for six doses, starting at day 1 after the second immunization. LV-shBAFF treatment dramatically reduced the percentage of CII-immunized mice developing CIA by almost 50% and markedly delayed the onset of arthritis (Fig. 1 D and E). Only mild inflammation in synovium with normal joint structure was found in LV-shBAFF–treated joints (Fig. 1F). Accordingly, computed tomography (CT) scans did not detect any obvious narrowing of joint space and bone erosion as compared with lentiviral vector expressing shRNA for β-actin (LV-shAct)–treated controls (Fig. 1G and Movie S1 and Movie S2 online). This protective effect of LV-shBAFF could be observed even at day 200 after the second immunization (Fig. 1H). As a positive control, the lentivirus expressing BAFF gene (LV-BAFF) was constructed (Fig. S2 A–C online) and local administration of LV-BAFF for only three doses, starting at day 1 after the second immunization, markedly exacerbated arthritis progression with severe synovial hyperplasia, cartilage damage, and bone erosion (Fig. S2 D–F online). To evaluate whether LV-shBAFF can affect established arthritis, we treated inflamed limbs at a clinical score of 1.0 with LV-shBAFF, which rapidly suppressed arthritic progression with significantly ameliorated joint damage (Fig. 1 I and J). Moreover, local injection with lentivirus expressing Emerald-GFP (LV-EmGFP) showed no signs of lentivirus dissemination to other organs, including the spleen, liver, kidney, and brain (data not shown).

Fig. 1.
Lentivirus-mediated BAFF gene silencing inhibits arthritis development in CII-immunized mice. (A) Semiquantitative PCR analysis of BAFF, a proliferation-inducing ligand (APRIL), β-actin, and GAPDH mRNA levels of I13.35 cells at day 2 after transfection ...

BAFF Gene Silencing Inhibits the Generation of Th17 Cells in Arthritic Joint.

Consistent with the observation of ameliorated joint inflammation by local BAFF gene silencing, markedly reduced numbers of leukocytes were found in joint tissue and popliteal lymph nodes (PLNs) but not in spleen (Fig. 2 A–C). Multilineage analysis showed significantly reduced numbers of PCs, B cells, T cells, DCs, and macrophages in LV-shBAFF–treated joints and PLNs, whereas no obvious change was observed among most cell types, with the exception of DCs in the spleen (Fig. S3 A–C online). The numbers of CII-specific IgG-secreting cells were substantially reduced in the joints and PLNs of the LV-shBAFF–treated group (Fig. 2 D–F), which was in sharp contrast to the results of the LV-BAFF–treated group (Fig. 2 A–F; Fig. S3 A–C online).

Fig. 2.
BAFF gene silencing in the joint tissue of CIA mice inhibits generation of Th17 cells in situ. (A–C) Total cell numbers of various organs in PBS, lentiviral vector expressing shRNA for β-actin (LV-shAct), LV-shBAFF, LV-control vector (Ctl), ...

In the LV-shBAFF–treated group, we found significantly reduced numbers of IL-17–secreting cells in the joint and PLN by sixfold and twofold, respectively (Fig. 2 G–I), whereas flow cytometric analysis detected absence of Th17 cells in joints and a threefold reduction of Th17-cell frequencies in PLNs (Fig. 2J). Analysis of LV-shBAFF–treated joint tissue revealed significantly down-regulated mRNA levels of IL-17A and IL-17F. IL-6 and IL-23 were also markedly reduced in their expression (Fig. S4A online). Moreover, chemokine levels of CXCL12, CXCL13, and CCL19 were significantly down-regulated. As expected, the contrary results were observed in the LV-BAFF–treated group (Fig. 2 G–J; Fig. S4B online). IL-6 and TGF-β are critically involved in inducing Th17-cell differentiation from naïve CD4+ T cells, whereas IL-23 can promote expansion of Th17 cells (15). We furthermore found that BAFF overexpression with LV-BAFF treatment substantially increased numbers of IL-6–expressing cells in joint tissue by immunohistochemical studies (Fig. S4C online). Taken together, these findings indicate that local LV-shBAFF treatment leads to an inhibitory cytokine milieu that effectively inhibits Th17 cells in situ.

Lentivirus Targets DCs in Joint Tissue, and BAFF Gene–Silenced DCs Are Defective in Supporting Th17-Cell Differentiation.

To determine whether lentivirus targets DCs in vivo, we detected LV-EmGFP–infected CD11c+ DCs in joint tissue (Fig. 3A). In bone marrow–derived dendritic cells (BMDCs), we detected surface expressions of all BAFF receptors, BAFF-R, TACI, and BCMA (Fig. S5A), implicating a potential role for BAFF in regulating DCs. LV-shBAFF–transduced BMDCs (LV-shBAFF BMDCs) were found to display an immature phenotype and were defective in driving B-cell maturation, PC differentiation, and antibody production (Fig. 3 B–D; Figs. S5 and S6 online).

Fig. 3.
Lentivirus targets DCs in the joint and BAFF gene–silencing DCs are defective in driving Th17-cell differentiation. (A) Emerald (Em)-GFP detection and CD11c immunofluorescence staining of frozen ankle sections from mice that had received intra-articular ...

We next sought to determine the role of BAFF in DC-mediated Th17-cell differentiation (16). We cultured LPS-matured LV-shBAFF BMDCs with naïve CD4+ T cells (CD4+CD62LhiCD44loCD25) and found significantly reduced frequencies of Th17 cells and levels of IL-17 production (Fig. 3 E and F). Consistent with the observation of markedly reduced IL-6 production in LV-shBAFF BMDCs, replenishment of IL-6 in the cocultures restored the yield of Th17 cells. These data indicate that BAFF is involved in DC-mediated generation of Th17 cells.

IL-17 Is Critically Involved in BAFF-Mediated Exacerbation of CIA.

The inhibitory effects of LV-shBAFF treatment on Th17 cells in the joint tissue led us to investigate further the possible mechanisms by which BAFF modulates Th17 cells. We administered recombinant BAFF systemically to CII-immunized mice and found substantially increased Th17 cells in the spleen (Fig. 4A). Based on our observed effects of BAFF on exacerbating arthritis development (Fig. S1), we next sought to verify whether IL-17 is involved in mediating this effect. We injected recombinant BAFF systemically into CII-immunized IL-17−/− mice. BAFF treatment markedly increased the incidence of arthritis development in WT mice but had no apparent effect in IL-17−/− mice (Fig. 4B). To ascertain whether this is attributable to potentially defective humoral response in IL-17−/− mice, we found a significantly increased frequency of CII-specific antibody-secreting cells in their spleens on BAFF treatment (Fig. 4C), indicating that normal humoral responses can be elicited in these mutant mice. Together, these results suggest that IL-17 acts as a key effector cytokine for BAFF-mediated exacerbation of CIA development.

Fig. 4.
IL-17 is critically involved in BAFF-mediated exacerbation of CIA development. (A) Enzyme-linked immunospot (ELISPOT) results of Th17 cells from the spleens of CIA mice at day 20 after 10 consecutive doses of systemic BAFF treatment compared with control ...

We found that BAFF binds to both naïve and activated CD4+ T cells (Fig. S7 online) and therefore we examined whether BAFF can induce de novo differentiation of Th17 cells. Indeed, BAFF treatment did not induce Th17 cell differentiation from naive CD4+ T cells but increased both frequency of Th17 cells and expression of IL-17 only when TGF-β and IL-6 were present (Fig. S8 A–D). We also found that BAFF promoted proliferation of total CD4+ T cells and specifically enhanced the proliferation of IFN-γ+ and IL-17+ T cells but inhibited that of IL-4+ T cells (Fig. S8 F–G), demonstrating that BAFF preferentially drives the expansion of Th1 and Th17 lineages. These results are consistent with previous findings that BAFF augments Th1-associated inflammatory responses (17).


In this study, we describe the development of replication-defective LV-shBAFF gene silencing in treating experimental arthritis. Our local gene-silencing strategy effectively inhibits leukocyte infiltration, synovial hyperplasia, and joint destruction in CIA mice and is also capable of suppressing arthritic progression after its onset. Thus, these findings validate BAFF as a potential therapeutic target for the treatment of autoimmune arthritis.

Consistent with the well-recognized function of BAFF as a key survival factor for B-cell maturation and activation (18, 19), local BAFF gene silencing significantly reduces the number of anti-CII antibody–secreting PCs in targeted joint tissue. Recent studies have shown that BAFF can selectively enhance the survival of plasmablasts generated from memory B cells (20). It is also clear that the generation of Ig-secreting plasmablasts from memory B cells requires functional interactions among antigen-specific B cells, T cells, and DCs (21). Our previous studies have revealed that BAFF is involved in driving DC-mediated B-cell proliferation on anti-μ stimulation (3). Notably, DCs were found to produce high levels of BAFF during CIA development (3). Thus, the efficacy of local LV-shBAFF treatment in suppressing arthritis development could be attributed in part to the capability of lentivirus in targeting terminally differentiated cells that produce BAFF, such as DCs (22, 23), which is furthermore supported by our in vitro findings that BAFF gene silencing renders DCs with markedly reduced capacities in promoting PC generation and antibody production. The ectopic lymphoid-like follicles in the inflamed synovial tissue of RA patients often display GCs filled with aggregated B cells (24, 25). Although the pathogenic role of autoreactive B cells and autoantibodies in local joint tissue remains to be further established, there is strong evidence that B-cell depletion in inflamed synovium leads to markedly diminished inflammatory status of the transplanted RA synovial tissue in SCID mice (26). Moreover, B cells can produce a variety of cytokines, including IL-4 and IL-10, and regulate T cell activation and function, contributing to the antibody-independent effector functions of B cells in autoimmune diseases (27). Thus, our observed efficacy of BAFF gene silencing in targeting inflamed joint tissue furthermore supports an increasingly recognized central role of B cells in the immunopathogenesis of both RA and SLE, as demonstrated by the clinical benefits of B-cell–targeted therapies for these diseases (28).

Proinflammatory cytokines have been recognized as important mediators for RA pathogenesis (8). We show that administration of LV-shBAFF in the CIA mice suppresses the expression of proinflammatory cytokines, such as IL-6, IL-1β, and IL-23, in joint tissue, an effect possibly contributed predominantly by its inhibition on leukocyte infiltration. In addition, the significant down-regulation of chemokine levels in joint tissues after LV-shBAFF treatment may furthermore suppress the chemotaxis of both T and B cells, a notion reinforced by recent findings that CXCL13 and BAFF synergistically enhance B-cell chemotaxis (29). BAFF gene–silenced DCs show defective IL-6 production and display severely impaired capacity in inducing Th17-cell generation in vitro. These results are consistent with previous findings that APC-produced IL-6 is critically involved in driving Th17 cells to induce T cell reactivity in the SKG mice (30). Moreover, DBA/1J mice with inactivation of the IL-6 gene are completely protected from CIA, whereas IL-6−/− mice are incapable of developing a Th17 response (31, 32). Our findings that injection with recombinant BAFF-expressing lentivirus substantially increases the number of IL-6–expressing cells in the joint tissue furthermore supports BAFF as an important modulator for the cytokine milieu that affects the generation and function of Th17 cells in situ.

Th17 cells can recruit and activate inflammatory cells and have been recognized as a primary cause of bone destruction and inflammation in autoimmune diseases (33, 34). Our in vitro studies show that BAFF does not induce de novo Th17-cell differentiation from naïve CD4+ T cells but can preferentially promote Th17-cell proliferation and expansion. Furthermore, our findings that BAFF treatment shows no apparent effect on enhancing the CIA development in IL-17−/− mice support the notion that IL-17 serves as a key effector cytokine for BAFF-mediated proinflammatory effects during CIA pathogenesis. Thus, the efficacy of LV-shBAFF treatment could be partially explained by the direct effect of BAFF on promoting Th17-cell expansion.

Several therapeutic strategies, including pharmacological inhibition of PI3Kγ, have recently been explored to suppress synovial inflammation and joint damage in mouse models of RA, but the potential side effects derived from the systemic approach remain to be evaluated (9, 35). In this study, the local gene-targeting strategy does not affect the numbers and functions of B and T cells in the spleen (Fig. S3C and data not shown), which is in sharp contrast to the markedly reduced splenic T- and B-cell responses in TACI-Fc–treated mice (11), indicating that our current approach poses no apparent systemic immunosuppression. Although we have observed moderately reduced DCs in the spleen after LV-shBAFF treatment, further studies are required to determine whether this treatment affects the turnover of DCs in the spleen as well as the homeostatic maintenance of DCs in other peripheral tissues.

In conclusion, we have demonstrated that local delivery of lentivirus-mediated BAFF gene silencing can effectively ameliorate autoimmune arthritis without systemic immunosuppression. Our results also reveal the role of BAFF as a promoting factor for the expansion of Th17 cells. Thus, our strategy to silence BAFF expression locally may prove to be a promising approach for the effective treatment of RA.

Materials and Methods

Cell Lines and Mice.

Murine macrophage I13.35 cells were purchased from American Type Culture Collection. Transformed human embryonic kidney cells, 293FT, and human fibrosarcoma cells, HT1080, were from Invitrogen. DBA/1J and 129/Sv mice were obtained from the Jackson Laboratory. C57BL/6 mice were obtained from the Charles River Laboratory. IL-17−/− mice in the 129/Sv × C57BL/6 F1 hybrid background were obtained from Yoichiro Iwakura (University of Tokyo, Tokyo, Japan) (6). 129/Sv and C57BL/6 were crossed to give the F1 hybrid as a control for IL-17−/− mice. All mice were maintained in a pathogen-free animal facility at the University of Hong Kong. All experiments were approved by the institutional animal care and use committee.


The induction of CIA was described previously (3). Briefly, male DBA/1J mice at 10 wk of age were injected intradermally at the base of the tail with 200 μg of bovine CII (Chondrex) emulsified with complete Freund adjuvant. For IL-17−/− and 129/Sv X C57BL/6 control mice, immunization was carried out as previously described (6) by using chicken CII (Sigma-Aldrich). The arthritis severity was evaluated as previously described (3).


Mouse ankles were fixed in 10% buffered formalin in PBS overnight, decalcified in 10% formic acid in water for 24 h, embedded in paraffin, and sectioned at 5-μm thickness before staining with H&E. Immunohistochemical analyses were performed on paraffin sections where immunoreactivity of the tissue sample was improved after heat-induced epitope retrieval in a microwave processor. Slides were then placed in 3% H2O2 in water to inhibit endogenous peroxidase activity. After washing, slides were blocked with 20% normal goat serum in Tris-buffered saline for 30 min before incubation with the primary antibody (biotinylated rat anti-mouse monoclonal antibodies for either BAFF or IL-6) overnight at 4°C. After washing, slides were incubated with peroxidase-conjugated streptavidin (StreptABComplex/HRP; Dako). After being incubated with 3, 3′-diaminobenzidine tetrahydrochloride (0.5 mg/ml), slides were counterstained with Mayer's hematoxylin and mounted in VectaMount (Vector Laboratories). Isotype-matched antibodies were used as a negative control.

CT Scan.

Micro-CT volumetric imaging was performed by using a cone beam scanner (uCT-20; SCANCO Medical) with a fixed x-ray fan beam of 7-μm spot size at 50 kVp and 160 mA. Integration time was at 140 msec, and slices scanned at high resolution (1024 × 1024 pixel matrix per slice) and size of 25 μvoxel.

Transfection of I13.35 Cells with siRNA.

As previously described (36), 20 nM siRNA (sequences listed in Table S1) was transfected into I13.35 cells by using lipofectamine 2000. Growth medium was replaced after 4 h of transfection, and cells were harvested 48 h after transfection for RNA synthesis.

Construction of Recombinant Lentiviral Vectors and Virus Packaging.

BLOCK-iT Lentiviral RNAi Expression System (25–0663; Invitrogen) was used for the construction of the lentiviral expression construct according to the manufacturer's instructions. Short pairs of sense and antisense DNA oligo encoding a sense-loop-antisense sequence to BAFF or the control β-actin gene were synthesized for the validated corresponding siRNAs, and sequences are listed in Table S1. The complementary DNA oligos were annealed and ligated to the BLOCK-iT U6 RNAi Entry vectors (Invitrogen) and subcloned into the plenti6/BLOCK-iT-DEST vector (Invitrogen). For cloning of the BAFF gene, the coding sequence of BAFF was amplified from C57BL/6 mouse spleen cDNA by using primers listed in Table S1 and ligated to pENTR/D-TOPO (K2400–20; Invitrogen) before being transferred to lentiviral vector pLenti6/V5-DEST (V496–10; Invitrogen). All the cloned sequences were confirmed by DNA sequencing (Genome Research Center, University of Hong Kong).

The recombinant lentiviral vectors and pLenti6.3/V5-GW/EmGFP vector were individually cotransfected with ViraPower packaging mix (11668–027; Invitrogen) by using lipofectamine 2000 and packaged into pseudotyped lentivirus by using 293FT cells. Viral supernatant was filtered through a 0.45-μm filter and concentrated by ultracentrifugation at 20,000 × g for 1 h at 4°C and stored at −80°C.

Real-Time PCR.

Quantitative real-time PCR was performed with the SYBR green two-step qRT-PCR kit with ROX (Invitrogen) as previously described (36). The following conditions were used: 95°C for 2 min, 40 cycles of 95°C for 30 s, and 60°C for 30 s. The threshold cycle (CT) of gene products was determined and set to the log linear range of the amplification curve and kept constant. Relative expressions were calculated with normalization to β-actin values by using the comparative CT method, where fold difference = 2−(CT of gene of interest‖ CT of β actin) = 2−ΔCT. The sequences of the primers were designed to span at least one intron and are listed in Table S2.

Cell Cultures.

BMDCs were generated from BM cell suspensions as previously described (37). Cocultures of naïve CD4+ T cells with BMDCs were performed as previously described (16). Naïve CD4+ T cells (2.5 × 105) were cultured for 2 days with 1 × 105 BMDCs that matured with 100 ng/ml LPS in 1 ml of complete growth medium for 1 day. Mouse CD3/CD28 T cell expander (5 μl of Dynabeads; Invitrogen) was added, and exogenous cytokines used were TGF-β (1 ng/ml) and IL-6 (20 ng/ml) (R & D Systems).

Naïve CD4+ T cell cultures were performed as previously described (38). Briefly, 1 × 105 CD4+ naïve T cells isolated from spleens of C57B/6 mice were cultured in 1 ml of complete growth medium in the presence of 2.5 μl of Dynabeads mouse CD3/CD28 T cell expander for 2 days. As indicated, cultures were supplemented with TGF-β (1 ng/ml) and IL-6 (20 ng/ml) in the absence or presence of BAFF (50, 100, or 200 ng/ml).

Total CD4+ T cell cultures were set up by first labeling 1 × 105 CIA mouse splenic CD4+ T cells with 2 μM CFSE and were incubated with 1 × 106 irradiated APCs and 100 μg/ml CII in 1 ml of complete growth medium.

Statistical Analysis.

Data are presented as the mean ± SD. Means were compared with the Student's t test where appropriate for statistical analysis. P values of <0.05 are considered statistically significant.

Additional Materials and Methods can be found at SI Materials and Methods online.

Supplementary Material

Supporting Information:


We thank Sarah Chan, Cherry Lo, and Eric Poon for technical assistance and Dr. Keung Leung for CT scan analysis. We are grateful to Dr. Yoichiro Iwakura (University of Tokyo) for providing IL-17−/− mice. This work was supported by grants from Hong Kong Innovation and Technology Commission Grant ITS/073/06 (to L.L.) and the Research Grant Council of Hong Kong Grant HKU7608/06M (to L.L.).


The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/cgi/content/full/0806044105/DCSupplemental.


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